Expression of transgenes in cells is becoming an important therapeutic approach for a variety of conditions. For example, in adoptive immunotherapy, human T lymphocytes are engineered by gene transfer to express chimeric antigen receptors (CARs) specific for surface molecules expressed on tumor cells. Chimeric receptors are synthetic receptors that include an extracellular ligand binding domain, most commonly a single chain variable fragment of a monoclonal antibody (scFv) linked to intracellular signaling components, most commonly CD3ζ alone or combined with one or more costimulatory domains. Other examples of conditions treated with transgene modified cells include thalassemia, hemophilia, myocardial infarction, and severe combined immunodeficiency. However, a major issue remains with obtaining stable expression of transgene expression at levels comparable to endogenous genes. There is a need to identify compositions and methods for selecting and/or detecting cells that express transgenes at high levels.